1. Technical Field of the Invention
This invention relates to the synthesis of nanoparticles, also known as nanocrystallites or quantum dots. In particular, quantum dots are synthesized using new solvents. More particularly, a nucleation step is separated from the growth process by quenching the reaction solution with cold solvent.
2. Description of the Related Art
Semiconductor nanocrystals are of great importance for both fundamental and application purposes. The optical and electrical properties of quantum dots are size-dependent, which makes them attractive for many applications. (See, for example, Alivisatos, A. P., Science 271, 933-937 (1996); Murray et al., Annual Review of Materials Science 30, 545-610 (2000).) Intensive research has been done in this area in the last few decades. Bukowski and Simmons (Critical Reviews in Solid State and Materials Science 27, 119-142 (2002)) describe many potential applications for semiconductor nanocrystals, such as quantum dot infrared photodetectors, quantum dot lasers, and photovoltaic devices.
Murray et al. reported the synthesis of high-quality CdE (E=S, Se, Te) nanocrystals through the rapid injection of organometallic reagents (trioctylphosphine selenide [TOPSe], or trioctylphosphine telluride [TOPTe] with a cadmium source (dimethylcadmium) [Me2Cd] dissolved in trioctylphosphine [TOP]) into a hot coordinating solvent (trioctylphosphine oxide [TOPO]) followed by size selective precipitation (Journal of the American Chemical Society 115, 8706-8715 (1993)). (See also Katari, et al., Journal of Physical Chemistry 98, 4109-4117 (1994).)
In their work, Dabbousi et al. described the synthesis and characterization of a series of core-shell (CdSe)ZnS nanocrystals with CdSe cores diameter ranging from 2.3 to 5.5 nm. A quantum yield of (30-50%) and photoluminescence characterized by FWHM of less than 40 nm were achieved. The nanocrystals were synthesized via the pyrolysis of Cd(CH3)2 and TOPSe in a coordinating solvent (TOPO) (Journal of Physical Chemistry B 101 9463-9475 (1997)). See also U.S. Pat. No. 6,322,901 to Bawendi et al.
A systematic shape variation (nanorods, arrows, teardrops, and tetrapods) of nanocrystals has been achieved via synthesizing the nanoparticles in a mixture of hexylphosphonic acid and trioctylphosphine oxide. The ratio of surfactants, injection  volume, and monomer concentration are the main factors in nanoparticle shape control. Ostwald ripening occurs at low monomer concentrations, whereas at high monomer concentrations the differences in the growth rates of different faces may lead to different shape configurations (Manna et al., Journal of the American Chemical Society 122, 12700-12706 (2000)).
Peng and Peng further explored the temporal shape evolution of CdSe nanorods in trioctylphosphine oxide solvent. (Journal of the American Chemical Society 123, 1389-1395 (2001)). Nanocrystals grow exclusively along the c-axis of the wurtzite structure at high monomer concentrations, whereas, at intermediate monomer concentrations, the growth is simultaneously in three dimensions. On the other hand, at low monomer concentrations, the intraparticle diffusion on the surface of the nanocrystal decreases the aspect ratio. Moreover, at lower monomer concentrations Ostwald ripening takes place (Peng and Peng, (2001) and Peng et al., Journal of the American Chemical Society 120, 5343-5344 (1998)).
Duan and Lieber reported the synthesis of a broad range (III-V, II-VI, and IV-IV) of semiconductor nanowires with diameters ranging from three to tens of nanometers and lengths up to tens of micrometers using laser-assisted growth (Advanced Materials 12, 298-302 (2000)).
In an important development, CdSe nanocrystals have been reported to be synthesized using safer and lower cost alternative routes. Fatty acids, amines, phosphine oxides, and phosphonic acids are among those solvent systems used. Moreover, CdO was found to be a more desirable source of cadmium for the synthesis of nanocrystals compared to Cd (CH3)2 (Qu et al., Nano Letters 1, 333-337 (2001)). Different varieties of elongated shapes were synthesized using these alternative routes. The magic size nuclei and the concentration of the remaining monomers after the initial nucleation stage are claimed to be the determining factors of the shape configuration of the nanostructured particles (Peng and Peng, Journal of the American Chemical Society 123, 1389-1395 (2001)). In another study, Qu and Peng concluded that during the growth of nanocrystals under a given set of initial conditions, photoluminescence quantum yield increases to a maximum value (bright point) before it gradually decreases. This maximum value is said to be a signature of an optimal surface structure (Journal of the American Chemical Society 124, 2049-2055 (2002)).
In an effort to set the basis for developing photoluminescence-based labeling reagents, the chemistry of the CdSe dendron-nanocrystals was investigated. Dendron ligands were used to stabilize CdSe and Au nanocrystals. Hydrophilic organic dendron ligands were designed to stabilize semiconductor and noble metal nanocrystals by binding onto the surface  of the nanocrystal (Wang et. al, Journal of the American Chemical Society 124, 2293-2298 (2002)).
Recently, Yu and Peng successfully synthesized II-VI semiconductor nanocrystals in non-coordinating solvents. CdS, CdSe, and ZnSe nanocrystals were synthesized in octadecene (ODE) with oleic acid (OA) as a ligand to stabilize the nanocrystals and the cationic precursors. The growth rate of nanocrystals was found to be dependent on the concentration of oleic acid in ODE. As the concentration of OA in ODE increases, the growth rate increases (Angewandte Chemie International Edition 41, 2368-2371 (2002)).
The choice of solvent for the synthesis of nanocrystals is of great importance, particularly for the purpose of scaling up the production of nanoparticles to achieve commercial nanomanufacturing and mass production. Nanocrystals are currently commercially available (e.g., from Quantum Dot Corp., Hayward, Calif.; Evident Technologies, Troy, N.Y.) but are relatively expensive. The present invention provides new methods to achieve low cost using commercially available solvents for the synthesis of semiconductor nanocrystals, which may have a significant effect on the economic feasibility of commercializing the synthesis of nanoparticles. DOWTHERM®, which is a well known heating fluid and a registered trademark of Dow Chemical Company, a Michigan corporation, has now been used successfully as a solvent to synthesize CdSe quantum dots. Moreover, phenyl ether and biphenyl, which are the constituents of Dowtherm A, were used individually as solvents for the synthesis of CdSe nanocrystals, as was THERMINOL® 66 heat transfer fluid, manufactured by Solutia, Inc.
Another feature of the present invention is the separation of nucleation and growth through quenching the synthesis by adding a certain amount of the relatively cold solvent (about room temperature) into the reaction solution immediately after injecting the selenium solution.